HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS EXPRESS HIGH-AFFINITY RECEPTORS FOR FACTOR XA

The binding of [I-125]-factor Xa to human umbilical vein endothelial c ell (HUVEC) monolayers was studied. At 7 degrees C, [I-125]-factor Xa bound to a single class of binding sites with a dissociation constant value of 6.6 +/- 0.8 nM and a binding site density of 57,460 +/- 5,200 sites/cell (n = 3). Association and dissociation kinetics were of a p seudo-first order and gave association and dissociation rate constant values of 0.15 x 10(6) M-1 s(-1) and 4.0 x 10(-4) s(-1), respectively. [I-125]-factor Xa binding was inhibited by factor Xa but was not affe cted by factor X, thrombin or monoclonal antibodies against factor V, antithrombin-III or tissue factor pathway inhibitor (TFPI) but was inh ibited by an antibody specific for the effector cell protease receptor -1 (EPR-1), a well-known receptor of factor Xa on various cell types. [I-125]-factor Xa binding to HUVEC was not affected by various inhibit ors of factor Xa such as DX 9065, pentasaccharide-antithrombin-III or TFPI. Factor Xa increased intracellular free calcium levels and phosph oinositide turnover in endothelial cells and, when added to HUVEC in c ulture, factor Xa was a potent mitogen, stimulating an increase in cel l number at a 0.3 to 100 nM concentration. HUVEC-bound factor Xa promo ted prothrombin activation in the presence of factor Va only. This eff ect was inhibited by both indirect and direct inhibitors of factor Xa. These findings indicate that HUVEC express functional high affinity r eceptors for factor Xa, related to EPR-1, which may be of importance i n the regulation of coagulation and homeostasis of the vascular wall. (C) 1997 Wiley-Liss, Inc.